Design and Fabrication of Opto-electronic Devices
A) Energy Conversion
Solution processable organic and hybrid semiconducting materials have emerged as alternatives to the traditional semiconductors due to their appealing features for roll-to-roll printing technology. In ORaCEL, we pursue fundamental research on understanding the processing, and optoelectronic properties of bulk heterojunction and perovskite solar cells.
Organic solar cells
With their efficiency exceeding 16%, organic bulk heterojunction (BHJ) solar cells are quickly approaching commercialization. ORaCEL researchers have made crucial contributions over the last decade through the development of high-efficiency solar cells, and investigation of fundamental factors influencing the generation, transport, and collection of photo-generated charges in operational solar cells. In particular, the Ade research group (Physics, NCSU) is well-known for investigating the phenomenological structure-function relations using Grazing Incidence Wide Angle X-ray Scattering (GIWAXS), Resonant Soft X-ray Scattering (R-SoXS) and Polarized Soft X-ray Scattering (P-SoXS) to probe the morphology of the thin films of organic semiconductors under various conditions. The group’s research has resulted in multiple reports in high impact factor journals including Nature Photonics, Nature Materials, and Joule. ORaCEL researchers also investigate charge generation dynamics through transient optical measurements (Gundogdu research group at Physics, NCSU), mechanical properties of BHJ devices (O’Connor research group at AME, NCSU), correlation of material dielectric properties with device performance (So research group at MSE, NCSU).
Recently, Researchers from NC State and UNC-Chapel Hill have launched a project to develop next-generation greenhouses with built-in solar cells that make use of the entire spectrum of solar light. BHJ solar cells generating energy from the sunlight that is not used by plants are integrated with greenhouses. We envision a new, zero-energy farming system that drastically improves the efficiency of land use and water consumption. And making greenhouses more effective across a broader range of climates would allow farms to be located next to urban centers. This unique project involves the synthesis of innovative materials (You group, UNC-Chapel Hill), development of flexible, transparent, and large area BHJ solar cells (Ade and O’Connor group, NCSU), plant engineering (Sederoff research group at Biology, NCSU), and economic analysis (DeCarolis research group at CCEE, NCSU). This research topic is currently funded by NSF.
Perovskite solar cells
Perovskite solar cells have demonstrated great potential for energy generation. Having recognized the unique advantage of this material, ORaCEL researchers have made great thrive in understanding the morphology of this material under various processing conditions. In particular, the Amassian research group (MSE, NCSU) is known in unraveling the morphological developments of perovskite films and solutions in in-situ x-ray based tools such as GIWAXS. This group’s recent findings have been reported in various high impact journals such as Nature, Joule, and Advanced Materials.
B. Light Emitting Diodes, and Photodetectors
Organic light emitting diodes (OLEDs) have recently made their way into commercialization as a backlight for displays in electronic devices. In displays, OLEDs provide high-quality images, lower power consumption, better durability, flexibility, and transparency. So research group (MSE, NCSU) has long been one of the champions in pushing OLED research for commercialization. With more than 80 patents, Dr. So develops OLEDs from polymers, and small molecules, while the group also develops perovskite, and quantum dots-based light emitting diodes.
Research on photodetectors has been part of ORaCEL research for the last few years. Infrared, visible, and multispectral photodetectors are important components for sensing, security and electronics applications. O’Connor research group (AME, NCSU) focuses on developing polymer-based photodetectors. Their recent work on an all-polymer photodetector appears in Advanced Optical Materials.
So group (MSE, NCSU) develops infrared sensing photodetectors from inorganic nanocrystals (NCs). The group recently demonstrated solution‐processed inorganic UV‐visible short‐wave‐infrared photodetectors with light sensitivity from 350 nm to 2000 nm using highly monodispersed large PbS NCs. These devices show detectivity values over 1 × 1011 Jones from 350 nm to 2000 nm, and a maximum detectivity value of 1.2 × 1012 Jones at 1800 nm. The group also reported on air‐stable multispectral solution‐processed inorganic double heterostructure photodetectors in Advanced Functional Materials.
Spintronics ― an idea of not only using the charge but also the spin of electrons ― shares many analogues with electric charges in the operations of conventional electronic devices. The recent emergence of novel spintronics studies focuses on the generation, transmission, and control of a ‘pure spin current’ through spin-orbit effects by converting the pure spin current into the charge current or vice versa. While an efficient spin-to-charge interconversion in a variety of semiconducting and metallic systems has been long sought, their conversion efficiency induced by the bulk spin-orbit effect remains moderate. An interface-driven spin-orbit coupling mechanism ― The Rashba effect ― shows great promises due to its unprecedented spin-to-charge interconversion efficiency [Fig. 1]. This efficient spin-to-charge conversion stemming from the Rashba state has revolutionized the field of spintronics by entailing a wide exploration of novel Rashba-type materials, and by envisioning the building of next-generation pure spin current generators, switches, detectors, and memories.
Hybrid Metal Halide (HMH) is a new class of synthetic semiconductors prepared by low-temperature solution processing with a large chemical and structural ‘universe’ benefitting from the synthetic versatility of their molecular cations. While the family of HMHs has shown remarkable performance in photovoltaic and optoelectronic applications, their rich spintronic functionalities have yet to be utilized. Given their large spin-orbit coupling induced by the heavy metal atoms in their hybrid framework, HMHs are prime candidates to explore the Rashba splitting state for an efficient charge-spin interconversion with complementary electronic, photonic, and compositional functionalities.
Sun’s research group at NC State (Physics) focuses on unraveling the predicted Rashba splitting state in bulk 3D and 2D HMH materials and manipulating the Rashba-induced spin-to-charge conversion using state-of-the-art spin-pumping techniques that can take advantage of electronic, photonic, and synthetic interfaces. The group’s research aims to address grand challenges at the frontier of spintronic applications using solution-processed organic semiconductors and HMH materials, as reflected in the group’s recent publications in Nano Letters, and Nature Communications.
D. Textile Electronic systems
With organic semiconductors offering features for developing flexible electronics, ORaCEL is making efforts in developing flexible, textile electronics. Dr. Jur (Textiles, NCSU) leads the Nano-EXtended Textiles (NEXT) research lab in the Wilson College of Textiles, which aims to leverage nanotechnology materials and processing methods in to impart new characteristics to polymer films and fibers, with a specific focus on electronic and electro-optical modifications. Such an investigation requires an engineering design philosophy to be applied toward the ‘system-level’ development of the textiles and electronics to advance our technologies beyond the lab. Of particular interest to the NEXT team is the design of ‘smart’ textile platforms (wearable, internet of things) that enable improved materials integration of sensors, energy harvesting, energy storage, and communication devices. This includes studies studying materials that are commercially available from parallel supply chains (such as printed electronics) in order to understand opportunities for novel textile electronics. The NEXT team also explore cutting-edge nanomaterials and processes that is envisioned as being used in the next generation of textile electronics.
F. Field Effect Transistors
The organic electronics community has made tremendous efforts in realizing highly efficient flexible, transparent, cheap and large-area organic field effect transistors (OFETs). ORaCEL focuses on developing solution processed transistors aiming at unraveling fundamental material-function relationships and integrate OFETs in electronic devices. Ade group’s recent report in Nature Communications highlights conjugated polymer monolayer OFETs exhibiting charge carrier mobilities reaching 3 cm2 V−1 s−1. On the other hand, So research group (MSE, NCSU) makes great efforts in utilizing transistors in various electronic devices. A typical example of the group’s research has recently appeared in Nature Photonics, demonstrating high-gain vertical phototransistor with a perforated metallic source electrode having an EQE up to 1 × 105% and a detectivity of 1.2 × 1013 Jones. By incorporating a phosphorescent organic light-emitting diode in this phototransistor, an infrared-to-visible upconversion LEPT with a photon-to-photon conversion efficiency of over 1,000% was achieved.
ORaCEL transistor research also extends to perovskite-based systems. In a Nature Communications report, Amassian research group (MSE, NCSU) recently demonstrated transistors with micrometer-thin single crystals of methylammonium lead halide perovskites MAPbX3 (X = Cl, Br, I) with sub-nanometer surface roughness and very low surface contamination. The lateral and interfacial transport requirements of transistors make them particularly vulnerable to surface contamination and defects rife in polycrystalline films and bulk single crystals. Dr. Amassian et al.’s work led to ambipolar transistors with the record, room-temperature field-effect mobility up to 4.7 and 1.5 cm2 V−1 s−1 in p and n channel devices respectively, with 104 to 105 on-off ratio and low turn-on voltages.
Dougherty research group also studies organic nano-crystals in transistors, while Dinku investigates transistors with majority commodity insulators.